Torque limiting tools are widely used to tighten a fastener to a specific torque. Such tools are extensively used during surgical procedures, such as an orthopedic surgical procedure. For example, a torque limiting tool may be used to tighten a fastener that is used to secure an orthopedic implant or bone plate. As such, it is often important that the fastener is tightened to a specific torque. Over-tightening a fastener could result in damage to the orthopedic implant or bone plate. Likewise, a fastener that is not adequately tightened may result in undesirable movement of the implant or bone plate within the patient. Such movement of the orthopedic implant or bone plate may be adverse to a patient as the implant may move to a position that does not allow proper bone repair.
In addition, torque limiting tools may also be used to control the amount of torque that is applied during an orthopedic surgical procedure, such as the reaming of a bone. One such procedure is the reaming of the intramedullary canal of a long bone, such as a femur. During the procedure, a cutting tool that is positioned at the end of a shaft is inserted within the intramedullary canal. Torque applied to the shaft rotates the cutting tool so that tissue and bone material are removed from the canal. It is important that an appropriate amount of torque is applied to the shaft during this reaming procedure to ensure that the canal is appropriately reamed. Application of an excessive amount of torque however, may adversely result in damage to the shaft or the canal. Therefore, it is important that the amount of applied torque is limited to minimize the possibility of causing injury to the patient.
In some instances during an orthopedic reaming procedure, such as the reaming of the intramedullary canal, an obstacle may be encountered that causes the cutting tool to become immobilized therewithin. When a cutting tool becomes immobilized, continued application of torque may cause the shaft to fracture within the canal, which may result in injury to the patient. Furthermore, extraction of an immobilized cutting tool is particularly problematic as it may become necessary to further limit the amount of torque applied to the shaft to minimize potential damage to the patient. Therefore, a torque limiting tool is needed that applies torque to the shaft in two stages; one stage, having a first torque limit to allow application of an appropriate amount of torque to effectively ream a bone; and, a second stage, having a second torque limit that is different than the first torque limit that allows for the safe removal of the tool. More specifically, a tool is needed that is capable of providing a first maximum amount of torque during a first stage in which a bone is being reamed, and, in addition, is capable of limiting the application of torque to a second maximum torque amount that is less than the first maximum torque amount so that removal of the shaft and cutting tool is enabled.
The present invention provides a torque limiting tool having a mechanism that limits the application of torque at two separate stages. The tool of the present invention is designed so that application of torque in excess of a first pre-determined maximum torque threshold causes the mechanism to be modified so that further application of torque is limited to a second maximum torque threshold of a lesser amount. In contrast, most prior art torque limiting tools typically prevent the application of torque up to a single maximum torque limit and do not have the capability of limiting torque at different threshold amounts, such as at two different stages with the same mechanism.
For example, in the case of reaming an intramedullary canal, the first stage provides for the application of a first maximum amount of torque that allows for optimal removal of material therewithin. However, in the event that the reaming tool should become immobilized in, for example, the intramedullary canal, the torque limiting device of the present invention is designed to operate at a second stage in which the application of torque is limited to a second, lower torque threshold amount thereby minimizing potential bone damage during removal of the cutting tool.